1. Field of the Invention
The present invention relates generally to pattern generation for use in electron beam lithography and other applications.
2. Description of the Background Art
Electron-Beam Direct Write Lithography
As is understood in the art, a lithographic process includes the patterned exposure of a resist so that portions of the resist can be selectively removed to expose underlying areas for selective processing such as by etching, material deposition, implantation and the like. Traditional lithographic processes utilize electromagnetic energy in the form of ultraviolet light for selective exposure of the resist. As an alternative to electromagnetic energy (including x-rays), charged particle beams have been used for high resolution lithographic resist exposure. In particular, electron beams have been used since the low mass of electrons allows relatively accurate control of an electron beam at relatively low power and relatively high speed. Electron beam lithographic systems may be categorized as electron-beam direct write (EBDW) lithography systems and electron beam projection lithography systems.
In EBDW lithography, the substrate is sequentially exposed by means of a focused electron beam, wherein the beam either scans in the form of lines over the whole specimen and the desired structure is written on the object by corresponding blanking of the beam, or, as in a vector scan method, the focused electron beam is guided over the regions to be exposed. The beam spot may be shaped by a diaphragm.
EBDW is distinguished by high flexibility, since the circuit geometries are stored in the computer and can be optionally varied. Furthermore, very high resolutions can be attained by electron beam writing, since electron foci with small diameters may be attained with electron-optical imaging systems. However, it is disadvantageous that the process is very time-consuming, due to the sequential, point-wise writing. EBDW is therefore at present mainly used for the production of the masks required in projection lithography. In other words, EBDW lithography has the potential to achieve excellent resolution. However, EBDW has a traditional problem relating to its low throughput. For example, it may take ten to one hundred hours to inscribe an entire wafer using EBDW lithography. One previous approach to attempt to increase the throughput is by increasing the beam current. However, when the current density exceeds a certain threshold, electron-electron interactions cause the beam to blur.
Conventional Electron-Beam Projection Lithography
In electron-beam projection lithography, analogously to optical lithography, a larger portion of a mask is illuminated simultaneously and is imaged on a reduced scale on a wafer by means of projection optics. Since a whole field is imaged simultaneously in electron beam projection lithography, the attainable throughputs can be markedly higher in comparison with electron beam writers.
Projecting the electron-beam over a relatively wide area enables use of a high beam current while keeping the beam current density at a level consistent with minimal electron-electron interactions. For example, an area roughly 0.1 millimeters (mm) wide may be illuminated. That area is several orders of magnitude larger than a traditional EBDW system that focuses the beam into a much smaller spot, for example, with a spot size on the order of tens of nanometers (nm) wide.
A flood beam 0.1 mm wide would normally not provide a writing resolution sufficiently high for practical use in integrated circuit manufacturing. However, the system and method disclosed herein enables high-resolution writing by partitioning the flood beam into a multitude (for example, four million) of independently controllable beams.
While others have tried building multiple columns with multiple sources to achieve multiple beams, there are various difficulties in that approach. For example, there is the difficulty of making the multiple columns behave uniformly. The system and method disclosed herein may be implemented using a single column and a single source.
A conventional multi-beam system would require a large array of blankers to achieve a multitude of controllable beams from a single column, each blanker being a small and independently controllable element that can be switched on and off rapidly. However, it is quite problematic to build and control such a large array. For example, a blanker array for a conventional multi-beam system is not normally buildable using integrated circuits because such integrated circuits are opaque to electrons.
Another disadvantage of conventional electron-beam projection lithography systems is that a corresponding mask is necessary for each structure to be exposed. The preparation of customer-specific circuits in small numbers is not economic, because of the high costs associated with mask production.